1. Field of the Invention
This invention relates to a shortwave receiver and an antenna adaptor to be used for such a shortwave receiver.
2. Prior Art
Known shortwave receivers include those adapted to be connected to an external antenna, which may be a wire antenna or a rod antenna.
While a wire antenna is simply a wire that may be a lead wire and can only transfer the signal it receives to the receiver, it can show a satisfactory sensitivity when it has a sufficient length. A rod antenna can also show a satisfactory sensitivity when combined with a high frequency amplifier to make itself an active type antenna.
However, a wire antenna has no tuning effect and transfers any signal it receives regardless if the signal is a broadcast wave signal to be received properly by the receiver or not and hence is disadvantageous in terms of anti-jamming performance.
A rod antenna, on the other hand, will be short of sensitivity if combined with a high frequency amplifier when it is short and down-sized. A rod antenna showing a satisfactory level of sensitivity is inevitable rather long and large. Additionally, a rod antenna lacks directivity and is hence disadvantageous also in terms of anti-jamming performance.
In an attempt for bypassing the above identified problems, loop antennas have be used as external antennas. Since a loop antenna is basically a coil and hence of a tuning type, it has to be adapted to make its tuning frequency variable when used to receive shortwave broadcast signals over a wide frequency range.
Therefore, a loop antenna is normally connected to a tuning capacity element to form a tuning circuit, the capacity thereof being made variable for different meter bands (reception bands) to be used for signal reception. Then, it is provided with a high frequency amplifier that operates also as an output buffer so that the output signal of the tuning circuit is transmitted to the receiver by way of the high frequency amplifier.
Thus, a loop antenna can show a satisfactory level of sensitivity if it is down-sized. Additionally, it can be made to show a good anti-jamming effect because of its directivity and tuning ability. Still additionally, it can cover a wide frequency range as its tuning frequency is variable.
However, a loop antenna has to be operated in a cumbersome way to modify its tuning frequency in order to make itself adapted to a specific meter band.
Additionally, if the user of a shortwave receiver/recorder wants to sequentially record a plurality of broadcast programs transmitted in different respective meter bands by utilizing the automatic recording feature of the recorder that is provided with a timer, lie or she can record only the first program because the loop antenna is not adapted to tune in the transmission frequency of the second program. If the receiver/recorder has a memory scanning feature (a feature with which the receiver can sequentially receive programs of the transmission frequencies stored in the memory in advance), the feature is useless unless the tuning frequency of the loop antenna is correctly modified.
Therefore, the object of the present invention is to provide a shortwave receiver and an antenna adaptor that can bypass the above problem.
According to the invention, the above object is achieved by providing a shortwave receiver adapted to be used with an antenna module having:
a loop antenna;
a variable capacity diode connected in parallel to the loop antenna to form a tuning circuit for high frequency tuning operations; and
a high frequency amplifier for receiving the signal to be received selected by said tuning circuit;
said shortwave receiver comprising:
a receiving circuit for receiving the shortwave broadcast signal output from said antenna module;
a formation circuit for forming a tuning voltage to be applied to said variable capacity diode; and
a jack;
when using said antenna module, said forming circuit being controlled corresponding to the reception frequency of said receiving circuit so as to modify said tuning voltage to accommodate the meter band of the shortwave broadcast received by said receiving circuit and cause said tuning voltage to be output from said jack and applied to said variable capacity diode.
According to the invention, there is also provided an antenna adaptor to be used with an antenna module having:
a loop antenna;
a variable capacity diode connected in parallel to the loop antenna to form a tuning circuit for high frequency tuning operations; and
a high frequency amplifier for receiving the signal to be received selected by said tuning circuit;
and a shortwave receiver having:
a receiving circuit for receiving the shortwave broadcast signal output from said antenna module;
a formation circuit for forming a tuning voltage to be applied to said variable capacity diode; and
a jack;
said antenna module comprising:
a battery for supplying power to said high frequency amplifier;
a switching means for turning on/off the output voltage of said battery;
a plug adapted to be plugged into said jack; and
a cable connected to said plug;
when using said antenna module, said forming circuit being controlled corresponding to the reception frequency of said receiving circuit so as to modify said tuning voltage to accommodate the meter band of the shortwave broadcast received by said receiving circuit;
said switching means being turned on to apply the output voltage of said battery to said high frequency amplifier as operating voltage;
said tuning voltage being applied from said jack to said variable capacity diode by way of said plug and said cable;
the received signal output from said antenna module being supplied to said reception circuit by way of said cable and said plug.
Thus, according to the invention, when the reception frequency of the receiver is modified, the tuning frequency of the tuning circuit comprising a loop antenna is automatically modified accordingly so that the shortwave broadcast to be received can be tuned in with an enhanced level of sensitivity. Additionally, since the tuning circuit is used to provide the loop antenna with a selectivity for reliably detecting the reception frequency of the broadcast, no unnecessary and undesired broadcast wave signals are fed to the downstream to improve the anti jamming effect of the antenna. Still additionally, the anti-jamming effect of the loop antenna can be improved further by exploiting its directivity.
As the reception frequency of the receiver is shifted, the tuning frequency of the tuning circuit comprising the loop antenna is automatically modified to make the user free from the need of operating the loop antenna and provide him or her with an improved level of convenience.
When, the user wants to sequentially record a plurality of broadcast programs transmitted in different respective meter bands by means of a recorder, utilizing the automatic recording feature of the recorder that is provided with a timer, he or she can record the second and subsequent programs. If the receiver/recorder has a memory scanning feature (a feature with which the receiver can sequentially receive programs of the transmission frequencies stored in the memory in advance), the feature can be exploited by effectively using the loop antenna. Finally, since the receiving antenna is a loop antenna., it can be down-sized and made to show a high reception sensitivity if compared with a rod antenna or a wire antenna.